1. Field of the Invention
The field of the invention relates to digital encoding and decoding of information on a recording medium. The invention finds particular application in digital magnetic recording in order to optimize the information stored upon the magnetic media and in telecommunications transmissions in order to increase the effective data transfer rate of the information.
2. Description of the Prior Art
Various techniques for coding digital information have been developed since the inception of magnetic media to accommodate storage and retrieval of information in conformance with a pattern of flux reversals impressed upon recording media. While all digital encoding and decoding schemes have as their primary goal the storage of information upon the magnetic media, there are advantages to employing certain schemes over others based upon the efficiency of information storage. That is, some digital encoding schemes permit greater quantities of information to be stored than other schemes when normalized to a certain predetermined number of flux reversals.
For example, digital data may be encoded using the so-called frequency modulation or FM scheme. In the well known FM-encoding scheme, clock pulses are interspersed with data pulses. A clock pulse is written at the beginning of each bit cell and data pulses are either written (data "1") or not written (data "0") in the middle of a particular bit cell depending upon the information to be stored. Decoding is naturally the reverse of encoding. FM encoding is relatively inefficient with respect to the amount of media consumed to store a given amount of information. A method for clocking is required to implement FM-encoding as flux transitions must appear at fixed intervals, and the packing density of bits (i.e., flux transitions) is extremely high relative to the information stored.
As a second example, modified frequency modulation or MFM is well known in the art and enhances the efficiency of digital information storage compared with single density or FM encoding. In MFM encoding, data bits are either written (data "1") or not written (data "0") in the middle of a bit cell depending upon the information to be stored, and clock pulses are only written at the beginning of a bit cell if no data bit has been written in the previous bit cell and no data bit will be written in the present bit cell. By use of MFM, the packing density of flux reversals is significantly reduced for an equivalent amount of stored information relative to FM, because clock pulses are seldom written. However, for both FM and MFM, a phase locked oscillator (PLO) has been employed to separate clock pulses from data pulses within a pulse stream with the phase locked oscillator synchronized to some timing reference standard. Both FM and MFM are limited in the amount of data that can theoretically be stored within a given number of flux reversals. Also, both FM and MFM in prior art systems require the constant input from a phase locked oscillator for comparing to the data stream in order to decode clock pulses from data pulses (and conversely, the PLO is required in order to write the correct stream of clock pulses and data pulses in the data writing operation). That is, both FM and MFM-encoding schemes utilize the concept of a fixed duration "bit cell" whether an actual flux reversal or bit occurs within the bit cell or not.
Another digital encoding/decoding scheme is known in the art as "3PM." The 3PM code establishes a fixed interval which is thereafter divided into six subintervals. Under the rules of the code, two flux reversals are written within each grouping of six subintervals, but pulses may not be written closer than three subintervals apart. A single interval of six subintervals suffices under the code to store three bits. The 3PM code effectively increases the efficiency of data storage compared to MFM encoding by approximately 1.5 times. However, the 3PM code also requires a phase locked oscillator, because the placement of flux reversals upon the media must occur within predetermined intervals or windows.
Moreover, in FM, MFM or even the more modern 3PM digital encoding/decoding schemes, a fixed time interval is always required to store each bit of data. In FM and MFM encoding, three bit cells are required to store three bits. In the 3PM code, one major bit cell divided into six subintervals is required to store three bits. In any case, no more than three bits, in this example, may be stored in less than three bit cells for FM or MFM and one major interval for 3PM irrespective of the data being encoded.
The prior art in magnetic recording, coding and encoding, is characterized by the general concept of run length limited codes of which FM, MFM, and 3PM are particular types. Run length limited codes, in general, always require a clock, bit cell durations have an absolute minimum, and bit cell durations have an absolute maximum. Such run length limited codes have limited efficiency as measured by the number of flux reversals required to store a certain data stream and the efficiency of the code is basically independent of the data stream. In practice, the establishment of bit cells has required the universal use of phase locked oscillators in high density applications.
In the area of telecommunications, the technology can be generally divided into two categories: asynchronous communications and synchronous communications. Both asynchronous and synchronous communications are bit serial techniques, but synchronous communications are of relatively high speed. Typical high speed synchronous protocols are defined by the well-known BISYNC and SDLC standards. Asynchronous communications, on the other hand, are of relatively low speed. A typical application of low frequency asynchronous communication is a Western Electric 202 modem which attaches to a standard telephone line. The maximum data rate achievable is 1200 baud, or 1200 bits/second. Unconditioned telephone lines demand a bandwidth limitation of 2400 Hz due to the characteristics of the transmission medium.
As with magnetic media, synchronous telecommunications, and even asynchronous telecommunications, an increase in data storage efficiency or data transmission efficiency without exceeding the physical constraints of the storage and/or transmission medium is an ever-present goal. Although economically of tremendous importance, significant increases in data storage efficiency over MFM in magnetic recording media or significant increases in the data transfer rate over 1200 baud using an unconditioned telephone line have not been available.